Most analytical techniques used in industry require taking samples to the laboratory to be analyzed by time consuming procedures. For use in the field, e.g., on-site analysis, spectral analyzers have been gaining favor because of the potential speed of analysis and the fact that they often represent a non-destructive means of analyzing samples. Based on spectroscopy technology, it is possible not only to determine the characteristics of a sample surface, but often the constituent components beneath a sample surface.
Typically, in spectroscopic applications an optimal range of wavelengths is selected to irradiate a sample, where reflected or transmitted light is measured to determine the characteristics of the sample. Some samples, for example, are best analyzed using a near infrared spectrum of light while others are optimally analyzed using a range such as visible or mid infrared spectrum.
Many spectral analyzers utilize a narrow spot size to intensely irradiate a sample to be analyzed. Illuminating a sample with a highly intense incident light typically results in an easier collection of larger amounts of reflected light, thus improving system performance. Unfortunately, a narrow spot size can sometimes provide inaccurate measurements because a small spot may not be representative of the intended sample, particularly where the sample is heterogenous in nature, such as, for example, grains, seeds, powders or and other particulate or suspended analytes. A narrow spot may unduly heat the sample, affecting the nature of the spectra.
To illustrate, it has been long recognized that the value of agricultural products such as cereal grains and the like are affected by the quality of their inherent constituent components. In particular, cereal grains with desirable protein, oil, starch, fiber, and moisture content and desirable levels of carbohydrates and other constituents can command a premium price. Favorable markets for these grains and their processed commodities have therefore created the need for knowing content and also various other physical characteristics such as hardness and xe2x80x9ctest weightxe2x80x9d (bulk density). Accordingly, when a truck with a trailer load of grain arrives at a grain elevator, the elevator operator needs to obtain a good statistical sample of the grain in the truckload, and then measure the properties of the samples. From this sampling, the overall properties of the grain (such as protein, oil and moisture content) are estimated for the truckload. Fast measurement and immediate answers are desired so that the grain may be judged as acceptable or not, and if acceptable, directed to the proper storage location based on the measured characteristics. Current methods utilize a physical sampling probe, which is driven vertically down into the grain and mechanically or pneumatically withdraws samples from various depths. The withdrawn samples are then analyzed, e.g., by infrared techniques. However, the cost of labor and time for serially withdrawing individual samples and then processing the samples can limit the number of samples withdrawn from a given truckload of grain and therefore potentially hamper the ability to obtain good sampling statistics.
Another problem with on-site spectroscopic detection techniques can arise in situations where the analyte to be detected, e.g., fluids or particulates, is being transported across the field of vision of the spectral analyzer, such as in a chute or on a conveyor belt. For instance, an open fluid or particle xe2x80x9cstreamxe2x80x9d having a varying cross-sectional dimension can present difficulties where it is necessary that some portion of the spectral probe be positioned at a fixed distance from the surface of the stream. To illustrate, the truckload of grain referred to above may transported from the truck to locations within the elevator facilities on conveyor belts, in some cases at speeds as fast as 10 feet/second. The unevenness of the stream of grain on the belt can be problematic to positioning a spectroscopy probe at a constant fixed distance from a surface of the grain stream. On the other hand, inserting the probe into the stream to maintain a constant distance between the probe head and the grain being analyzed may cause unacceptable turbulence in the flow of particles or fluid.
Moreover, in certain instances the fluid or particle stream may be fast enough that difficulties are encountered in obtaining enough measurements for good statistical sampling, particularly where the particle or fluid stream is heterogeneous in composition. Returning again to the example of the grain elevator, many of the transport processes which may be amenable to spectroscopic detection from the standpoint of accessibility to the grain, e.g., for placement of infrared probes and the like, may in fact be less than ideal due to the speed with which the grain would be transported by the field of vision of the probe. Grain being unloaded from a truck, for example, may be unloaded though delivery chutes at a rate of tens of bushels per second. In view of the potential heterogeneity in the grain being monitored, and the speed with which the grain is moving, providing good statistical sampling of the quality of the grain by spectroscopic techniques using a probe positioned along the flow path can be impaired by the lack of time to get an adequate number of sample spectra.
The present invention relates to spectral analysis systems and methods for determining physical and chemical properties of a sample by measuring the optical characteristics of its transmitted and/or reflected light. In general, the systems and methods of the present invention are useful for examining the spectroscopic characteristics of materials, such as particles or liquids, though the systems may be used to characterize other materials such as suspensions of particles and even gases. In certain embodiments, it is especially advantageous to use the subject system in connection with non-uniform material, e.g. consisting of components of different compositions, because the system of the present invention does not require the samples to be homogeneous in order to achieve reliable results.
However, in addition to characterizing heterogeneous materials, the subject systems can also be used to ascertain whether or when a mixture or a stream of material is sufficiently homogeneous or fulfils certain specifications with regard to content and/or particle size.
One aspect of the present invention relates to an insertion probe system for spectral analysis of flowable materials, or other materials, including static materials, into which a probe can be inserted, for which internal spectroscopic sampling is desired. In such embodiments, the invention provides a spectral analysis system including a probe which can be inserted into, e.g., bins, bales, vats, blenders, silos, mixers, drums, flow streams, and the like, of granular, powder or liquid matter and suspensions.
In general, the probe may include a probe head having: (i) a light source arranged to irradiate a sample volume of the material proximate the probe head, which source may be a lamp or other radiation source disposed in the probe head or it may be the radiant end of an optical fiber or other waveguide delivering light from a source distal to the probe head; and (ii) an optical pick-up, arranged to receive light energy reflected or otherwise emitted from a sample in the irradiated sample volume. The light source provides a suitably broad bandwidth of light for irradiating the sample, and in certain preferred embodiments, simultaneously irradiates at multiple wavelengths. The light pick-up receives light reflected or emitted from a sample being irradiated, and is in optical communication with one or more detectors which measure the intensity of the light reflected or emitted by the sample in a wavelength-dependent manner. Where the detector is located distal to the probe head, the pick-up may be an aperture in the probe head connected with an optical fiber or other waveguide which communicates light reflected or emitted by the sample to the detector. Where the detector is proximal to the irradiated sample, as it may be if disposed in the probe head, the pick-up may simply be an aperture for permitting light being reflected by the sample to enter the probe head. The system can also include one or more signal processing circuits, such as in the form of a computation subsystem, for processing signals from the detector.
A salient feature to certain preferred embodiments of the subject insertion probe relates to the sample volume irradiated by the probe. As described in further detail below, the irradiated sample volume can be shaped to be circumferential, or at least substantially circumferential, to the light source, and preferably to the long (insertion) axis of the probe. For instance, the probe may irradiate a toroidal sample volume wrapping circumferentially around the light source. Moreover, the sample volume is preferably disposed 180xc2x0 to 360xc2x0 circumferentially around the light source, and more preferably 270xc2x0 to 360xc2x0, and even more preferably 360xc2x0 around the light source. In certain embodiments, the irradiation pattern provides for an irradiation surface area of about 10 times R2, and more preferably at least about 25 times R2, at least about 50 times R2, at least about 75 times R2 or even at least about 100 times R2, where R is the radius of the probe. By providing a larger sample volume, the advantages to such configurations of the system include the ability to collect data more likely to be statistically representative of a heterogeneous mixture and to get better signal-to-noise in the spectral analysis. Moreover, a larger sample volume permits a more efficient use of the light and helps to provide improved signal-to-noise.
Another aspect of the present invention relates to a variable surface probe system for spectroscopic analysis of a moving sample of a flowable material. In particular, the invention provides a spectral analysis system including a probe which can be variably positioned in contact with the moving surface of the material, or a fixed distance below the surface, without substantially disrupting the flow of the material. In such embodiments, the invention provides a spectral analysis system including a probe which can be inserted or placed on top of, e.g., moving material on a conveyor belt, grain belt, and the like.
In general, the probe may include a probe head having: (i) a light source arranged to irradiate the flowable material proximate the probe head, which source may be a lamp or other radiation source disposed in the probe head or it may be the radiant end of an optical fiber or other waveguide delivering light from a source distal to the probe head; (ii) an optical pick-up, arranged to receive light energy reflected or otherwise emitted from a sample in the irradiated sample volume; (iii) a planing element which permits the probe head to skim the surface of the flowing material when in contact; and, optionally, (iv) a constant force generator which applies a force to the probe head to maintain a constant amount of contact between the probe and the sample. The planing element of the probe may be, merely to illustrate, convex or concave such that when contacted with the surface of the moving material, e.g., at a shallow angle of attack, the planing element allows the probe to traverse the flowing material without creating significant turbulence in the material. The light source provides a suitably broad bandwidth of light for irradiating the sample, and in certain preferred embodiments, simultaneously with multiple radiation wavelengths. The light pick-up receives light reflected or emitted from a sample being irradiated, and is in optical communication with one or more detectors which measure the intensity of the reflected light, e.g., in a wavelength-dependent manner. Where the detector is located distal to the probe head, the pick-up may be an aperture in the probe head connected with an optical fiber or other waveguide which communicates light reflected or emitted by the sample to the detector. Where the detector is proximal to the irradiated sample, as it may be if disposed in the probe head, the pick-up may simply be an aperture for permitting light being reflected by the sample to enter the probe head. The system can also include one or more signal processing circuits, such as in the form of a computation subsystem, for processing signals outputted from the detector.
Still another aspect of the present invention relates to a multihead probe system for spectroscopic analysis of a moving sample of a flowable material. In such embodiments, the invention provides a spectral analysis system including a probe which can be inserted into a fast moving flow, e.g., a truck discharging its load at a grain elevator. It may be used in any granular solid or liquid or gas that moves through or along a passage, either enclosed or open. This could include manure, soil, sludge, mining materials, raw and fine chemicals, pharmaceuticals, food stuffs, waste materials, hazardous waste, petroleum and its products, commercial gaseous products, stack gases, etc.
In particular, the invention provides a spectral analysis system including a plurality of probe heads, e.g., which are simultaneously (relative to each other) able to irradiate and collect spectral information on the moving sample. In general, each of the plurality of probes may include a probe head having: (i) a light source arranged to irradiate the flowable material proximate the probe head, which source may be a lamp or other radiation source disposed in the probe head or it may be the radiant end of an optical fiber or other waveguide delivering light from a source distal to the probe head; and (ii) an optical pick-up, arranged to receive light energy reflected or otherwise emitted from a sample in the irradiated sample volume. Each light source provides a suitably broad bandwidth of light for irradiating the sample, and in certain preferred embodiments, the light sources may simultaneously irradiate the sample with multiple radiation wavelengths, e.g., each light source may provide light at a distinct wavelength. The light pick-up receives light reflected or emitted from a sample being irradiated, and is in optical communication with one or more detectors which measure the intensity of the reflected light, e.g., in a wavelength-dependent manner. Where the detectors are located distal to the probe head, the pick-up may be an aperture in the probe head connected with an optical fiber or other waveguide which communicates light reflected or emitted by the sample to the detector. Where the detector is proximal to the irradiated sample, as it may be if disposed in the probe head, the pick-up may simply be an aperture for permitting light being reflected by the sample to enter the probe head. The system can also include one or more signal processing circuits, such as in the form of a computation subsystem, for processing signals outputted from the detector.
Still another aspect of the invention relates to a probe system for spectroscopic analysis of a sample material that minimizes the effects of surface reflection on the spectral analysis of the sample thereby improving the spectral analysis. In such embodiments, the invention provides a probe system for spectral analysis in industrial, drug manufacturing, chemical and petrochemical settings and the like. In one particular embodiment, the probe is used in situations with sample materials having a large component of surface reflections relative to light paths passing through particles or a bulk of sample material in a diffuse, scattering path.
In particular, the invention provides a probe head for use with a spectrometer to analyze a material, the probe head having: (i) a light source arranged to irradiate a sample volume of the material proximate the probe head, which source may be a lamp or other radiation source disposed in the probe head; (ii) an optical pick-up, arranged to receive light energy reflected or otherwise emitted from the sample in the irradiated sample volume and transmit the emitted light to the spectrometer for analysis; (iii) an optical blocking element positioned within the optical path between the light source and the optical pick-up to force the optical path into the sample volume; and (iv) a reference shutter for selectively blocking light emitted from the irradiated sample volume from reaching the optical pick-up to facilitate calibration. The optical blocking element minimizes direct surface reflections from the sample or from components of the probe head, such as, for example, a sample window positioned in contact with or proximate the material, relative to light passing through and reflecting from the material within the sample volume to thereby improve the accuracy of the analysis of the material. The light source provides a suitably broad bandwidth of light for irradiating the sample, and in certain preferred embodiments, simultaneously with multiple radiation wavelengths. The light pick-up receives light reflected or emitted from a sample being irradiated, and is in optical communication with one or more detectors which measure the intensity of the reflected light, e.g., in a wavelength-dependent manner. Where the detector is located distal to the probe head, the pick-up may be an aperture in the probe head connected with an optical fiber or other waveguide which communicates light reflected or emitted by the sample to the detector. Where the detector is proximal to the irradiated sample, as it may be if disposed in the probe head, the pick-up may simply be an aperture for permitting light being reflected by the sample to enter the probe head. The system can also include one or more signal processing circuits, such as in the form of a computation subsystem, for processing signals outputted from the detector.
In one embodiment of the subject method, the composition of the inspected material can be quantified by detecting molecular vibrational modes characteristic of one or more constituents of the material, as for example proteins, lipids, fatty acids, etc. This aspect of the method comprises irradiating the sample with electromagnetic radiation, e.g., infrared radiation, e.g., preferably near infrared radiation, in a wavelength range which is converted by the sample into molecular vibrations, e.g., in the wavelength range of infrared radiation, and measuring at least one of an absorption or transmission of the electromagnetic radiation by the sample. Infrared radiation refers broadly to that part of the electromagnetic spectrum between the visible and microwave regions. This encompasses the wavelengths from about 700 nm to about 50,000 nm. Near infrared radiation includes wavelengths in the range of about 700-2500 nm. For instance, it has been discovered that protein levels in grains can be determined by measuring near infrared absorption at particular wavelengths. As used herein, the term xe2x80x9cnear infraredxe2x80x9d or xe2x80x9cnear IRxe2x80x9d is intended to encompass light in a spectrum ranging from about 700 to about 2500 nm, more preferably from about 1300 to about 2400, and, in some instances, most preferably from about 1400 to about 2200 nm.
In certain preferred embodiments, the subject systems and methods measure a spectral response to short wavelength, near infrared (NIR) radiant energy in the range 700-2500 nm, and even more preferably from 600 to about 1100 nanometers (nm). The system may also be set up to irradiate the sample in the visible spectrum, including wavelengths as low as about 400 nanometers (nm). The spectral response at shorter wavelengths helps in the modeling of proteins and other constituents in conjunction with the response at higher wavelengths, and be useful in those embodiments where grains or other protein-containing materials are being characterized.
Although the infrared spectrum is characteristic of the entire molecule, certain groups of atoms give rise to bands at or near the same frequency regardless of the structure of the rest of the molecule. It is the persistence of these characteristic bands that permits the practitioner to obtain useful structural information by simple inspection and reference to generalized charts of characteristic group frequencies. To illustrate, the conjugated diketone is a structure that is likely to be persistent irrespective of the length of a fatty acid. Furthermore, other chemical structures of proteins, fatty acids and other natural constituents have been determined that and are suitable for detection by infrared means.
Infrared radiation of frequencies less than about 100 cmxe2x88x921 (wavelengths longer than 10,000 nm) can be absorbed and converted by a constituent of the sample into energy of molecular rotation. This absorption is quantized; thus a molecular rotation spectrum can consist of discrete lines. Infrared radiation in the range from about 10,000-100 cmxe2x88x921 (1000 nm-10,000 nm) can be absorbed and converted by the sample into energy of molecular vibration. This absorption is also quantized, but vibrational spectra appear as bands rather than as lines because a single vibrational energy change can be accompanied by a number of rotational energy changes. The frequency or wavelength of absorption depends on the relative masses of the atoms, the force constants of the bonds and the geometry of the atoms in the fatty acid.
Band positions in infrared spectra are presented either as wavenumbers or wavelengths and are understood to be equivalent. The wavenumber unit (cmxe2x88x921, reciprocal centimeters) is used most often since it is proportional to the energy of the vibration and since most modern instruments are linear in the cmxe2x88x921 scale. Wavelength, xcex, is referred to herein in terms of micrometers (xcexcm, 10xe2x88x926 meters) or nanometers (nm, 10xe2x88x929 meters). Wavenumbers are reciprocally related to wavelength, e.g., 1/xcex.
Band intensities can be classically expressed either as transmittance (T) or absorbance (A), though for the purpose of this application both of will be understood as within the meaning of the term xe2x80x9cabsorbancexe2x80x9d or xe2x80x9cabsorptionxe2x80x9d. As used in the art, transmittance is the ratio of the radiant power transmitted by a sample to the radiant power incident on the sample, and absorbance is the logarithm, to the base 10, of the reciprocal of the transmittance (A=log10(1/T)). The term absorbance or absorption further include scattered light, such as measured in Raman spectroscopy.
Moreover, other forms of vibrational spectroscopy, such as Raman spectroscopy, can be used as part of the subject methods. The Raman vibrational spectrum of these molecules can consist of a series of sharp lines which constitute a unique fingerprint of the specific molecular structure. Raman spectroscopy presents a means of obtaining vibrational spectra, especially over optical fibers, with visible or near infrared light, and provides a viable alternative to infrared spectrophotometry for use in the subject methods. These wavelength regions are efficiently transferred without significant absorption losses over conventional optical fiber materials. In Raman spectroscopy, monochromatic light is directed onto a sample and the spectrum of the scattered light is determined. However, due to a very weak signal, the excitation light must be quite intense, though laser light sources are readily available. In addition, optical filtering is necessary to separate the weak scattered signal from the intense Rayleigh line.
In yet another embodiment of the subject method, the constituents of a sample are determined in the sample by detecting molecular electronic modes characteristic of such constituents. This aspect of the method includes irradiating the sample with electromagnetic radiation, e.g., ultraviolet-visible radiation, e.g., ultraviolet radiation, in a wavelength range converted by the sample into electronic vibrations/electron orbital transitions, e.g., in the wavelength range of 200-400 nm, e.g., at a wavelength of 275 nm and measuring the absorption of the electromagnetic radiation by the sample. In the ultraviolet and visible region of the spectrum, molecular absorption is dependent on the electronic structure of the molecule. Absorption of energy is quantized, resulting in the elevation of electrons from the ground state to higher energy orbitals in an excited state. For many electronic structures, the absorption does not occur in the readily available portion of the ultraviolet region.
There is, however, an advantage to the selectivity of ultraviolet absorption: characteristic groups can be recognized in molecules of widely varying complexities. As a large portion of a relatively complex molecule can be transparent in the ultraviolet region, a spectrum can be obtained similar to that of a much simpler molecule.
Wavelengths in the ultraviolet region of the spectrum are usually expressed in nanometers or angstroms (xc3x85). The near ultraviolet (quartz) region includes wavelengths of 200-380 nm. The atmosphere is transparent in this region and quartz optics may be used to scan from 200 to 380 nm. Atmospheric absorption starts near 200 nm and extends into the shorter-wavelength region (10-200 nm), which is accessible through vacuum ultraviolet spectrometry.
The total energy of a molecule is the sum of its electronic energy, its vibrational energy, and its rotational energy. Energy absorbed in the ultraviolet region produces changes in the electronic energy of the molecule. These transitions consist of the excitation of an electron from an occupied orbital (usually a non-binding p or binding xcfx80-orbital) to the next higher energy orbital (an antibonding, xcfx80* or "sgr"*, orbital). The antibonding orbital is designated by an asterisk.
Since ultraviolet energy is quantized, the absorption spectrum arising from a single electronic transition should consist of a single, discrete line. A discrete line is not obtained since electronic absorption is superimposed on rotational and vibrational sublevels. The spectra of simple molecules in the gaseous state consist of narrow absorption peaks, each representing a transition from a particular combination of vibrational and rotational levels in the electronic ground state to a corresponding combination in the excited state. At ordinary temperatures, most of the molecules in the electronic ground state will be in the zero vibrational level; consequently, there are many electronic transitions from that level. In molecules containing more atoms, the multiplicity of vibrational sublevels and the closeness of their spacing cause the discrete bands to coalesce, and broad absorption bands or xe2x80x9cband envelopesxe2x80x9d are obtained.
The principal characteristics of an absorption band are its position and intensity. The position of absorption corresponds to the wavelength of radiation whose energy is equal to that required for an electronic transition. The intensity of absorption is largely dependent on two factors: the probability of interaction between the radiation energy and the electronic system and the difference between the ground and the excited state. The probability of transition is proportional to the square of the transition moment. The transition moment, or dipole moment of transition, is proportional to the change in the electronic charge distribution occurring during excitation. Intense absorption occurs when a transition is accompanied by a large change in the transition moment. Absorption with xcex5max values greater than 104 is high-intensity absorption; low-intensity absorption corresponds to xcex5max values less than 103.
Accordingly, the subject method relies on optically detecting individual chemical groups of a constituent of a sample which have been determined to be reliable as indicators for quantitatively determining the level of the constituent in the sample.
In one embodiment, the method comprises utilizing one of the subject systems for illuminating (e.g., irradiating) the sample at a plurality of discrete wavelengths, e.g. selected from the infrared, visible or ultraviolet spectrum. In certain embodiments, the wavelengths the sample is irradiated with include at least one sample wavelength and one reference wavelength. The sample wavelength is defined as being a wavelength for detecting a chemical feature whose existence is dependent on the presence of a constituent in the sample. The reference wavelength, on the other hand, is selected as a frequency which is not absorbed by the sample in a manner dependent on the presence of the constituent. Measurements of the intensity of transmitted, absorbed, or reflected light at such wavelengths are taken, and an analysis of transmittance, absorbance, or reflectance ratios for various wavelengths is performed.
In preferred embodiments, the reference wavelength is closely spaced and can be chosen so as to provide a xe2x80x9cbaselinexe2x80x9d for determining the intensity of the peak of interest, such as the band intensity of a peak arising due to the constituent. Changes in the ratios can be correlated from the sample wavelength, which obviously will vary with the state amount of the constituent in the sample, and the second (reference) wavelength, which is sufficiently removed from the sample wavelength so that measurements of light absorption at this second wavelength is relatively insensitive to the concentration of the constituent, and yet which is sufficiently close to the first wavelength to minimize interference from scattering effects and the like. Typically, the window bracketing these closely spaced wavelengths will be less than about 300 nm and preferably less than about 60 nm wide and, in some instances, more preferably less than about 30 nm wide. The reference wavelength can be chosen so as to detect a chemical feature which remains relatively unchanged (e.g. does not change in significant manner) as the normal makeup of the sample changes, or can be selected as a wavelength which does not correspond to any sharp absorption bands but which provides baseline correction to compensate for convoluted or xe2x80x9crollingxe2x80x9d baselines.
As will be understood, there are a wide variety of materials for which the systems and methods of the present invention can be used for characterization. Without intending to be limiting, exemplary materials include:
vegetable foods, such a wheat, corn, rye, oats, barley, soybeans, amaranth, triticale, and other grains, rice, coffee and cocoa, which may be in the form of whole grains or beans, or a ground or comminuted product (analysis for protein, starch, carbohydrate and/or water), seeds, e.g. peas and beans, such as soybeans (analysis for protein, fats and/or water), products mainly consisting of or extracted from vegetable raw materials, such as snacks, dough, vegetable mixtures, margarine, edible oils, fibre products, chocolate, sugar, syrup, lozenges and dried coffee extract (powder/granulate),
animal foodstuffs, such as dairy produce, e.g. milk, yogurt and other soured milk products, ice cream, cheese (analysis for protein, carbohydrate, lactose, fat and/or water), meat products, e.g. meat of pork, beef, mutton, poultry and fish in the form of minced or emulgated products (analysis for protein, fat, water and/or salts) and eggs, which foodstuffs may be present in a completely or partly frozen condition,
fermentation broths, such as alcoholic beverages, e.g. wine or beer,
fodder, e.g. pellets or dry/wet fodder mixtures of vegetable products, fats and protein-containing raw materials, including pet food,
manure and compost, including composting garbage, grass clippings,
pharmaceutical products, such as tablets, mixtures, powders, creams and ointments,
biological samples including, for example, biological fluids such as blood, urine, spinal fluid, saliva, etc, and tissue samples, and
technical substances, e.g. wet and dry mixtures of cement and mortar, plastics, e.g. in granular form, mineral materials, such as solvents and petro-chemical products, e.g. oils, hydrocarbons and asphalt, solutions of organic or inorganic substances, e.g. sugar solutions, glue and epoxies, and
liquids with light scattering properties in suspension, slurries, fluidized materials including both solid and liquid and similar entities.
The components comprising the systems of the present invention are preferably integrated into a single unit, e.g., to create either a portable spectral analyzer or one which is readily disposed along a path of a moving material.